Mechanisms of forming damascene interconnect structures

ABSTRACT

A method of forming a conductive structure includes forming a first opening and a second opening in a dielectric layer on a substrate, wherein the first opening is narrower than the second opening. The method further includes depositing a diffusion barrier layer to line the first opening and the second opening. The method further includes forming a metal layer over the diffusion barrier layer to fill at least portions of the first opening and the second opening, wherein a maximum thickness of the metal layer in the first opening is greater than a maximum thickness of the metal layer in the second opening.

PRIORITY CLAIM

The present application is a divisional of U.S. application Ser. No. 13/434,691, filed Mar. 29, 2012, which claims priority of U.S. Provisional Application No. 61/562,705, filed Nov. 22, 2011, which are incorporated herein by reference in their entireties.

TECHNICAL FIELD

This invention relates generally to metallization of an integrated circuit (IC) device, and more specifically to the formation methods of interconnect structures.

BACKGROUND

Semiconductor devices use interconnect, which includes metal lines and contacts/vias, to provide connection between active and/or device devices with external contacts. Typically, the metal patterns of different metallization layers are electrically interconnected by vias. Semiconductor devices with interconnect circuits, according to current technology, may comprise eight or more levels of metallization to satisfy device connection and geometry requirements. There are challenges in forming interconnect for advanced device technologies.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of an interconnect structure, in accordance with some embodiments.

FIGS. 2A through 2K are cross-sectional views of intermediate stages in the making of interconnect structures, in accordance with some embodiments.

FIG. 3 is a cross-sectional view of an interconnect structure with dual damascene structures, in accordance with some embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. It is understood that those skilled in the art will be able to devise various equivalents that, although not explicitly described herein, embody the principles of the present invention.

A common method for forming metal lines is known as “damascene.” Generally, this process involves forming an opening in the dielectric interlayer, which separates the vertically spaced metallization layers. The opening is typically formed using conventional lithographic and etching techniques. After an opening is formed, the opening is filled with copper or copper alloys to form a metal line and/or a via. Excess metal material on the surface of the dielectric interlayer is then removed by chemical mechanical polish (CMP). Although copper has low resistivity and high reliability, copper still suffers from electro-migration (EM) and stress-migration (SM) reliability issues as geometries continue to shrink and current densities increase. Various approaches are thus explored to solve these problems.

FIG. 1 is a cross-sectional view of an interconnect structure 15, in accordance with some embodiments. Two copper lines 2 and 4 with a diffusion barrier layer 6 are formed in a low-k dielectric layer 14 adjacent to each other and are insulated from each other by low-k dielectric layer 14. Metal caps 10 and 12, which are formed of materials having fewer electro-migration issues in comparison with copper, are formed on copper lines 2 and 4, respectively. Metal caps 10 and 12 may be formed by electroless plating or selective chemical vapor deposition (CVD), in accordance with some embodiments. The formation of metal caps greatly improves the reliability of the integrated circuit by reducing the surface migration of the copper lines. It has been found that under stressed conditions, the mean time to failure (MTTF) of the illustrated interconnection structure is about ten times longer than that of an interconnect structure having no metal caps. Part of the reason for the improvement is the reduction of electro-migration. With the metal caps, stress-induced void formation is also significantly reduced. Such improvement is especially important for interconnect structures with fine feature sizes and high current flow because the stress is higher for such structures.

However, the introduction of metal caps can potentially generate other problems. Metal caps are typically formed on copper lines, thus increasing the height of the conductive materials. For example, the formation of metal caps 10 and 12 increases the height of the conductive materials from H′ to H. The parasitic capacitance between copper lines 2 and 4 (as well as the conductive materials surrounding copper lines 2 and 4) forms a parasitic capacitor, and the capacitance is proportional to the cross-sectional area of lines 2 and 4. Therefore, the formation of metal caps causes the parasitic capacitance to be H/H′ times the capacitance with no metal caps formed. As a result, resistive-capacitive (RC) delay of the integrated circuit is increased.

An additional effect caused by the formation of metal caps 10 and 12 by electroless plating or selective CVD is an increase in leakage current. As shown in FIG. 1, portions of metal caps 10 and 12 extend from copper lines 2 and 4 over and beyond top edges of diffusion barrier layer 6, in some embodiments. The extended portions E₁ and E₂ increase the leakage current between metal caps 10 and 12. As is known in the art, leakage currents and parasitic capacitance are more significant when the copper lines are close to each other. Such issues become more pronounced for advanced technologies with finer feature sizes and smaller pitches.

Further, during the formation of the metal caps 10 and 12, metal residues, such as residues R₁, may remain on the surface of low-k dielectric layer 14 and cause shorting and/or leakage. FIG. 1 also shows that an etch stop layer 16 may be deposited between the low-k dielectric layer 14 and an upper dielectric layer 18, in accordance with some embodiments. The protruding metal caps 10 and 12 would affect the film profile of the etch stop layer 16 and cause topographical issue, which could affect patterning of upper layers. In order to reduce parasitic capacitance, leakage current, shorting between neighboring conductive features, topographical effect, and to improve EM and SM performance, new mechanisms of forming interconnection structures are needed.

FIGS. 2A through 2K are cross-sectional views of intermediate stages in the making of interconnect structures, in accordance with some embodiments. FIG. 2A illustrates the formation of trenches 22 and 24 in a dielectric layer 20, which is an inter-metal dielectric (IMD) layer. The dielectric layer 20 is on a substrate 21, which may have active devices, such as transistors, etc., passive devices, such as resistors and others, interconnect structures, and insulating layers. In some embodiments, a hard mask (or etch stop) layer 23 is formed on dielectric layer 20 to assist the formation of trenches 22 and 24. The hard mask layer 23 may be made of SiN, SiON, or a combination thereof. The hard mask layer 23 may also be a metal hard mask.

In some embodiments, dielectric layer 20 has a dielectric constant (k value) lower than about 3.5. Suitable materials for the low-k dielectric material may include, but are not limited to, doped silicon dioxide, fluorinated silica glass (FSG), carbon-doped silicon dioxide, porous silicon dioxide, porous carbon-doped silicon dioxide, SiLK™ (an organic polymeric dielectric distributed by Dow Chemical of Michigan), Black Diamond (a product of Applied Materials of Santa Clara, Calif.), Xerogel, Aerogel, amorphous fluorinated carbon, Parylene, bis-benxocyclocutenes (BCB), polyimide, polynoroboneses, benzocyclocutene, PTFE, porous SiLK, hydrogen silsesquioxane (HSQ), methylsilsesquioxane (MSQ), and/or combinations thereof. The low-k dielectric material may be deposited by a chemical vapor deposition (CVD), plasma enhanced CVD (PECVD), or spin-on process.

The low-k dielectric could also be an extreme low-k dielectric (ELK). The ELK material may have a dielectric constant of less than about 2.5. Exemplary ELK materials include porous low-k materials. In some embodiments, the ELK is a silicon oxide based low-k material having a porous structure, which is adapted to a porogen-doped SiCO-based material by incorporating a porogen (or a porogen material) into a carbon-doped oxide dielectric. Other materials may also be used.

FIG. 2B illustrates a blanket formation of a diffusion barrier layer 28, which covers the sidewalls and bottoms of trenches 22 and 24. Diffusion barrier layer 28 may be formed of a material comprising titanium, titanium nitride, tantalum, tantalum nitride, ruthenium, ruthenium nitride, titanium compound, tantalum compound, cobalt, cobalt compound, manganese, manganese compounds, and combinations thereof. The formation methods may include physical vapor deposition (PVD), atomic layer deposition (ALD), chemical vapor deposition (CVD), and other commonly used methods.

FIG. 2B also shows a conductive layer 30 being formed on diffusion barrier layer 28. Conductive layer 30 may include copper or copper alloys, although other materials such as aluminum, tungsten, silver, and combinations thereof, can also be used. The conductive layer 30 may be formed by various applicable methods, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), etc. In some embodiments, after conductive layer 30 is deposited, substrate 21 is heated to allow conductive layer 30 on a top surface substrate 21 to flow into the remaining trenches 22 and 24, as shown in FIG. 2C. During the reflow process, the thickness T₁ of conductive layer 30 on the top surface 23 is reduced and the thicknesses T₂ and T₂′ of conductive layer 30 in the recess regions are increased due to the addition of the reflowed conductive layer 30. Due to narrower width of trench 24 than trench 22, thickness T₂ is higher than thickness T₂′. Increasing the substrate temperature increase the surface mobility of the conductive layer 30. In some embodiments, the temperature of the substrate is raised to be in a range from about 150° C. to about 450° C. to assist reflow of the conductive layer 30. In some other embodiments, the substrate temperature is maintained at a reflow temperature during the deposition of the conductive layer. Under such circumstance, the deposition and reflow of the conductive layer 30 are performed simultaneously.

The deposition and reflow of the conductive layer 30 are repeatedly multiple times, if they are performed separately, until a target thickness T of the conductive layer is reached, as shown in FIG. 2D, in accordance with some embodiments. However, if the deposition and reflow of the conductive layer 30 are performed simultaneously, the process is performed until the target thickness T is reached in the narrow trench 24. In some embodiments, the width of the narrow trench 24 is in a range from about 10 nm to about 100 nm. In some embodiments, the thickness T in the narrow trench 24 is in a range from about 10 nm to about 100 nm. After the targeted thickness T is reached, the diffusion barrier layer 28 on the top surface 25 of substrate 21 is removed. For example, a dry etching may be used. Prior to etching the diffusion barrier layer 28, substrate 21 could be patterned with a photoresist to expose the regions the diffusion barrier layer 28 need to be removed and to cover the remaining surfaces. FIG. 2E shows substrate 21 after the diffusion barrier layer 28 on surface 25 is removed, in accordance with some embodiments. In some embodiments, there is residual conductive layer 30 on surface 23 of diffusion barrier layer 28. The residual conductive layer 30 is removed prior to diffusion barrier layer 28 on surface 25 being removed. An etch process for etching the material of the conductive layer 30 may be used. The etch process may be dry or wet.

After the barrier layer 28 on surface 25 is removed, a removal process 210 is used to remove a portion of dielectric layer 20, as shown in FIG. 2F in accordance with some embodiments. The removal process 210 may be a chemical-mechanical polishing (CMP) process or an etching process. The thickness removed (T_(R)) is in a range from about 10 nm to about 60 nm, in some embodiments. The thickness removed (T_(R)) may also be larger than 60 nm. In some embodiments, the dielectric layer 20 is removed until the dielectric layer 20 is leveled with the conductive layer 30 in the narrow trench 24. In some other embodiments, the dielectric layer 20 is removed until the dielectric layer 20 is leveled with the conductive layer 30 in the wide trench 22. In some other embodiments, the removal process 210 is not needed. In some embodiments, a recess D₁ in the wide trench structure 22 is in a range from about 0 nm to about 60 nm. In some embodiments, D₂ (recess in the narrow trench structure 24) is less than D₁, or D₂<D₁.

Afterwards, a dielectric layer 40 may be deposited over the structures in FIG. 2E or structures in FIG. 2F after removing a portion of the dielectric layer 20, in some embodiments. The dielectric layer 40 is deposited to form next level of interconnect. Materials suitable to form dielectric layer 20 may also be used for dielectric layer 40. In some embodiment, an etch stop layer 41 is formed between the dielectric layer 40 and dielectric layer 20, as shown in FIG. 2G in accordance with some embodiments. Etch stop layer 41 is similar to the etch stop layer 16 described above. For example, the etch stop layer may be made of SiC. In some embodiments, the thickness of the etch stop layer 41 is in a range from about 5 nm to about 50 nm. Additional processing operations may be performed to form additional interconnect structures.

In some embodiments, after the deposition and reflow of the conductive layer 30 are performed to reach the targeted thickness T, as described in FIG. 2D above, a metal cap layer 50 may be deposited on substrate 21. The metal cap layer 50 is conductive and may include material(s) such as cobalt, nickel, tungsten, molybdenum, silicon, zinc, chrome, ruthenium, manganese, boron, phosphorus, nitrogen, and combinations thereof. The metal cap layer 50 may be a composite layer comprising more than one layer. A metal cap layer 50 has low resistivity and is a good diffusion barrier for copper. Also, the metal cap layer 50 should not form (or does not easily form) a complex or a compound with copper. Metal cap layer 50 has low reactivity with copper. Any materials having such characteristics may be used to form the metal cap layer 50. In some embodiments, the thickness of the metal cap layer 50 is in a range from about 0.5 nm to about 20 nm. The metal cap layer 50 may be deposited by PVD, CVD, ALD, or other applicable methods or processes.

After the metal cap layer 50 is deposited, a second conductive layer 55 is deposited, as shown in FIG. 2I in accordance with some embodiments. The second conductive layer 55 may be made of copper or copper alloys, although other materials such as aluminum, tungsten, silver, and combinations thereof, can also be used. The second conductive layer 55 may be formed by various applicable methods, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), electro-chemical plating, etc. In some embodiments, the second conductive layer may be deposited by the same method as the first conductive layer 30. In some embodiments, the thickness of the second conductive layer 55 is in a range from about 50 nm to about 1000 nm (measured on a field surface). FIG. 2H illustrates the metal cap layer 50 covering the substrate surface and the recess regions in trenches 22 and 24 and also the second conductive layer 55 over the metal cap layer 50, in accordance with some embodiments.

Afterwards, a removal process 220 may be performed to remove excess conductive layer 55, the metal layer 50, and diffusion barrier layer 28 on substrate surface 25, in some embodiments. The removal process 220 may be a CMP process or an etching process. In some embodiments, more than one removal processes are needed to remove the different layers on substrate surface 25. In some embodiments, the removal process also removes a portion of dielectric layer 20. FIGS. 2J and 2K show a cross-sectional view of substrate 21 after the removal process(es) is completed, in some embodiments. FIG. 2J shows that the removal process 220 leaves the second copper layer 55 in both trenches 22 and 24. In contrast, FIG. 2K shows that the etching process 220 removes the second copper layer 55 completely from the narrow trench 24 and leave a portion in trench 22. In some embodiment, the recess D₃ in the wide trench structure 22 is in a range from about 1 nm to about 50 nm. In some embodiment, the recess D₄ in the narrow trench structure 24 is less than D₃, or D₄<D₃.

The decision of using which interconnect structures in FIG. 2F (without metal cap), FIG. 2J, or FIG. 2K depends on the application. For applications with relatively low current, where leakage is not a significant issue, interconnect structures in FIG. 2F may be used. For applications with higher current, structures in FIGS. 2J and 2K may be used. The metal cap layers in the metal interconnect helps reducing copper diffusion. The wider interconnect structure 281 in FIGS. 2J and 2K have a second conductive layer 55 not covered by the metal cap layer 50. For wider interconnect structures, the current density is lower than the narrower structures, such as structure 280. As a result, the risk of leakage and/or electro-migration (EM) is very small or none. Therefore, a portion of the second conductive layer 55 not covered by the metal cap layer 50 does not cause leakage for wide structures. Leaving a second conductive layer 55 on the narrower structure 280 is acceptable for some applications. Further, leaving the second conductive layer 55 over the metal cap layer 50 allows process and/or electrical tunings.

In the embodiments provided in the preceding paragraphs, single damascene structures and processes are used to explain the mechanisms for forming the structures. One skilled in the art will realize that the teaching is readily available for forming a contact and/or via structure and dual damascene structures. The structures described in FIGS. 1, 2A-2K are trenches. However, they also can be contacts or vias. In some embodiments, the widths of the contacts or vias are in a range from about 5 nm to about 100 nm. FIG. 3 illustrates an interconnect structure 300 comprising dual damascene structures, in accordance with some embodiments. Similarly, in the embodiment shown in FIG. 3, metal cap 50′ is formed on via-metal structures 322 and 324. The wide structure 322 has a second conductive layer 55′, which is similar to the second conductive layer 55.

The mechanisms of forming an interconnect structures described above involves using a reflowed conductive layer. The reflowed conductive layer is thicker in smaller openings than in wider openings. The mechanisms may further involve forming a metal cap layer over the reflow conductive layer, in some embodiments. The interconnect structures formed by the mechanisms described have better electrical and reliability performance. For example, the parasitic capacitance, leakage current, shorting between neighboring conductive features and/or topographical effect can be reduced. In addition, the EM and SM performances can be improved.

One aspect of this description relates to a method of forming a conductive structure. The method includes forming a first opening and a second opening in a dielectric layer on a substrate, wherein the first opening is narrower than the second opening. The method further includes depositing a diffusion barrier layer to line the first opening and the second opening. The method further includes forming a metal layer over the diffusion barrier layer to fill at least portions of the first opening and the second opening, wherein a maximum thickness of the metal layer in the first opening is greater than a maximum thickness of the metal layer in the second opening.

Another aspect of this description relates to a method of forming a conductive structure. The method includes forming a first opening and a second opening in a dielectric layer on a substrate, wherein the first opening is narrower than the second opening. The method further includes depositing a diffusion barrier layer to line the first opening and the second opening. The method further includes depositing a conductive layer over the diffusion barrier layer in the first opening, in the second opening and outside of the first opening and the second opening, wherein the deposited conductive layer has a first thickness. The method further includes performing a thermal process on the conductive layer, wherein performing the thermal process on the conductive layer reduces a thickness of the conductive layer outside the first opening and the second opening to be less than the first thickness, and performing the thermal process on the conductive layer increases a thickness of the conductive layer in the first opening and in the second opening to be greater than the first thickness.

Still another aspect of this description relates to a method of forming a conductive structure. The method includes forming a first opening and a second opening in a dielectric layer on a substrate, wherein the first opening is narrower than the second opening. The method further includes depositing a continuous diffusion barrier layer to line the first opening and the second opening. The method further includes depositing a continuous conductive layer over the diffusion barrier layer, wherein the deposited conductive layer has a first thickness. The method further includes heating the continuous conductive layer, wherein heating the conductive layer reduces a thickness of the conductive layer outside the first opening and the second opening to be less than the first thickness, and heating the conductive layer increases a thickness of the conductive layer in the first opening and in the second opening to be greater than the first thickness. The method further includes depositing a cap layer over the continuous conductive layer, wherein a top surface of the cap layer in the first opening is farther from a top surface of the substrate than a top surface of the cap layer in the second opening. The method further includes depositing a second conductive layer over the cap layer.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. 

What is claimed is:
 1. A method of forming a conductive structure, comprising: forming a first opening and a second opening in a dielectric layer on a substrate, wherein the first opening is narrower than the second opening; depositing a diffusion barrier layer to line the first opening and the second opening; and forming a metal layer over the diffusion barrier layer to fill at least portions of the first opening and the second opening, wherein a maximum thickness of the metal layer in the first opening is greater than a maximum thickness of the metal layer in the second opening, and forming the metal layer comprises: maintaining the substrate at a reflow temperature during an entirety of the forming of the metal layer over the diffusion barrier layer, wherein the diffusion barrier layer outside of the first opening and the second opening is exposed by the reflow of the metal layer.
 2. The method of claim 1, wherein the metal layer includes copper.
 3. The method of claim 1, wherein the maintaining the substrate at the reflow temperature comprises maintaining the substrate at a temperature in a range from about 150° C. to about 450° C.
 4. The method of claim 1, further comprising: depositing a metal cap layer over the metal layer.
 5. The method of claim 4, further comprising: depositing another metal layer to fill a remaining portion of the first opening and the second opening.
 6. The method of claim 5, further comprising: performing a removal process to remove the other metal layer, the metal cap layer, and the diffusion barrier layer above the dielectric layer.
 7. The method of claim 4, wherein the metal cap layer has a thickness in a range from about 0.5 nm to about 20 nm.
 8. The method of claim 4, wherein the metal cap layer comprises at least one of cobalt, nickel, tungsten, molybdenum, silicon, zinc, chrome, boron, phosphorus, nitrogen, or combinations thereof.
 9. The method of claim 1, wherein the first opening has a width in a range from about 5 nm to about 100 nm.
 10. The method of claim 1, wherein forming the metal layer comprises forming the metal layer in the second opening having a substantially uniform thickness.
 11. A method of forming a conductive structure, comprising: forming a first opening and a second opening in a dielectric layer on a substrate, wherein the first opening is narrower than the second opening; depositing a diffusion barrier layer to line the first opening and the second opening; depositing a conductive layer over the diffusion barrier layer in the first opening, in the second opening and outside of the first opening and the second opening, wherein the deposited conductive layer has a first thickness; and performing a thermal process on the conductive layer, wherein the performing the thermal process on the conductive layer forms a top surface of the conductive layer in the second opening, wherein the top surface of the conducive layer at the second opening extends across and inside an entirety of the second opening.
 12. The method of claim 11, wherein performing the thermal process on the conductive layer comprises heating the substrate to a temperature in a range from about 150° C. to about 450° C.
 13. The method of claim 11, wherein performing the thermal process on the conductive layer is performed simultaneously with depositing the conductive layer.
 14. The method of claim 11, wherein performing the thermal process on the conductive layer comprises increasing a thickness of the conductive layer in the first opening to have a first maximum thickness, and increasing a thickness of the conductive layer in the second opening to have a second maximum thickness less than the first maximum thickness.
 15. The method of claim 11, further comprising repeating depositing the conductive layer and performing the thermal process on the conductive layer.
 16. A method of forming a conductive structure, comprising: forming a first opening and a second opening in a dielectric layer on a substrate, wherein the first opening is narrower than the second opening; depositing a continuous diffusion barrier layer to line the first opening and the second opening; depositing a continuous conductive layer over the diffusion barrier layer, wherein a maximum thickness of the deposited conductive layer has a first thickness; heating the continuous conductive layer, wherein heating the continuous conductive layer removes the continuous conductive layer from at least a portion of the diffusion barrier layer in the first opening or in the second opening; depositing a cap layer over the continuous conductive layer, wherein a top surface of the cap layer in the first opening is farther from a top surface of the substrate than a top surface of the cap layer in the second opening; and depositing a second conductive layer over the cap layer.
 17. The method of claim 16, further comprising removing the cap layer and the second conductive layer outside the first opening and the second opening.
 18. The method of claim 16, wherein depositing the cap layer comprises covering an entire top surface of the continuous conductive layer in the first opening with the cap layer.
 19. The method of claim 16, wherein depositing the second conductive layer comprises depositing the second conductive layer having a maximum thickness in the second opening greater than a maximum thickness in the first opening.
 20. The method of claim 16, further comprising removing the diffusion barrier layer outside the first opening and the second opening. 